SharedPreferences数据存储

SharedPreferences简介：适用于保存少量数据，且这些数据的格式都是基本类型的值。保存数据基于XML文件存储的key-value键值对数据。SharedPreferences对象本身只能获取数据，而不支持存储和修改，存储和修改是通过SharedPreferences.edit()获取的内部类接口Editor对象实现。SharedPreferences本身是一个接口，应用程序无法创建SharedPreferences实例，只能通过Context提供的getSharedPreferences(String name, int mode)方法来获取SharedPreferences实例。
Editor基础方法

• putString：保存一个键值对
• remove：删除一个键值对
• clear：清空SP数据
• commit：同步提交修改数据
• apply：异步提交修改数据

一、SharedPreferences基本使用

public class MainActivity extends AppCompatActivity {


    @SuppressLint("CommitPrefEdits")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //MODE_PRIVATE数据只能被本应用程序读、写
        //MODE_WORLD_READABLE数据能被其他应用程序读，但不能写
        //MODE_WORLD_WRITEABLE数据能被其他应用程序读，写
        SharedPreferences sharedPreferences = getSharedPreferences("name", MODE_PRIVATE);
        SharedPreferences.Editor editor = sharedPreferences.edit();
        //存储数据
        editor.putString("key", "value");
        editor.apply();
        //获取数据
        sharedPreferences.getString("key", "");

        //获取其他应用的SharedPreferences数据
        Context otherAppContent = null;
        try {
            //设置我们要调用数据的应用包名
            otherAppContent = createPackageContext("com.example.datastorageframe", CONTEXT_IGNORE_SECURITY);
        } catch (PackageManager.NameNotFoundException e) {
            e.printStackTrace();
        }
        assert otherAppContent != null;
        @SuppressLint("WorldReadableFiles")
        SharedPreferences otherSharedPreferences = otherAppContent.getSharedPreferences("test", MODE_WORLD_READABLE);
        otherSharedPreferences.getString("name", "");
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
    }
}

创建SharedPreferences工具类

public class SharedPreferencesUtils {
    public static final String name = "test";
    private static SharedPreferencesUtils instance;


    public static SharedPreferencesUtils getInstance() {
        if (instance == null) {
            synchronized (SharedPreferencesUtils.class) {
                if (instance == null) {
                    instance = new SharedPreferencesUtils();
                }
            }
        }
        return instance;
    }

    /**
     * 保存数据，修改数据
     *
     * @param key
     * @param value
     * @param <V>
     */
    public <V> void setValue(@NonNull String key, V value) {
        SharedPreferences sp = MyApp.getInstance().getSharedPreferences(name, Context.MODE_PRIVATE);
        SharedPreferences.Editor editor = sp.edit();
        if (value instanceof String) {
            editor.putString(key, (String) value);
        } else if (value instanceof Integer) {
            editor.putInt(key, (Integer) value);
        } else if (value instanceof Long) {
            editor.putLong(key, (Long) value);
        } else if (value instanceof Boolean) {
            editor.putBoolean(key, (Boolean) value);
        } else if (value instanceof Float) {
            editor.putFloat(key, (Float) value);
        }
        editor.commit();
    }

    /**
     * 读取数据
     *
     * @param key
     * @param defaultValue
     * @param <V>
     * @return
     */
    public <V> V getValue(@NonNull String key, V defaultValue) {
        SharedPreferences sp = MyApp.getInstance().getSharedPreferences(name, Context.MODE_PRIVATE);
        Object value = defaultValue;
        if (defaultValue instanceof String) {
            value = sp.getString(key, (String) defaultValue);
        } else if (defaultValue instanceof Integer) {
            value = sp.getInt(key, (Integer) defaultValue);
        } else if (defaultValue instanceof Long) {
            value = sp.getLong(key, (Long) defaultValue);
        } else if (defaultValue instanceof Boolean) {
            value = sp.getBoolean(key, (Boolean) defaultValue);
        } else if (defaultValue instanceof Float) {
            value = sp.getFloat(key, (Float) defaultValue);
        }
        return (V) value;
    }

    /**
     * 清空数据
     */
    public void clearData() {
        SharedPreferences.Editor editor = MyApp.getInstance().
                getSharedPreferences(name, Context.MODE_PRIVATE).edit();
        editor.clear();
        editor.commit();
    }
}

优点：SharedPreferences对象与SQLite数据库相比显得格外轻量级，免去了创建数据库，创建表，写SQL语句等诸多操作，相对而言更加方便，简洁。
缺点：只能存储boolean，int，float，long和String五种简单的数据类型。无法进行条件查询等。

二、源码解析

（1）getSharedPreferences源码

    @Override
    public SharedPreferences getSharedPreferences(String name, int mode) {
        //判空处理
        if (mPackageInfo.getApplicationInfo().targetSdkVersion <
                Build.VERSION_CODES.KITKAT) {
            if (name == null) {
                name = "null";
            }
        }

        File file;
        synchronized (ContextImpl.class) {
            if (mSharedPrefsPaths == null) {
                mSharedPrefsPaths = new ArrayMap<>();
            }
            //根据名字取文件
            file = mSharedPrefsPaths.get(name);
            if (file == null) {
                //根据名字创建文件
                file = getSharedPreferencesPath(name);
                //保存到ArrayMap中
                mSharedPrefsPaths.put(name, file);
            }
        }
        //根据文件获取sp对象
        return getSharedPreferences(file, mode);
    }

    @Override
    public SharedPreferences getSharedPreferences(File file, int mode) {
        //sp实现类
        SharedPreferencesImpl sp;
        synchronized (ContextImpl.class) {
            //根据包名获取ArrayMap<File, SharedPreferencesImpl>
            final ArrayMap<File, SharedPreferencesImpl> cache = getSharedPreferencesCacheLocked();
            sp = cache.get(file);
            if (sp == null) {
                checkMode(mode);
                if (getApplicationInfo().targetSdkVersion >= android.os.Build.VERSION_CODES.O) {
                    if (isCredentialProtectedStorage()
                            && !getSystemService(UserManager.class)
                                    .isUserUnlockingOrUnlocked(UserHandle.myUserId())) {
                        throw new IllegalStateException("SharedPreferences in credential encrypted "
                                + "storage are not available until after user is unlocked");
                    }
                }
                //实例化sp，并保存到缓存中
                sp = new SharedPreferencesImpl(file, mode);
                cache.put(file, sp);
                return sp;
            }
        }
        if ((mode & Context.MODE_MULTI_PROCESS) != 0 ||
            getApplicationInfo().targetSdkVersion < android.os.Build.VERSION_CODES.HONEYCOMB) {
            //如果是多进程模式需要重新读取文件
            sp.startReloadIfChangedUnexpectedly();
        }
        return sp;
    }

（2）SharedPreferencesImpl源码

    SharedPreferencesImpl(File file, int mode) {
        mFile = file;
        //备份file
        mBackupFile = makeBackupFile(file);
        mMode = mode;
        mLoaded = false;
        mMap = null;
        mThrowable = null;
        startLoadFromDisk();
    }

    private void startLoadFromDisk() {
        synchronized (mLock) {
            mLoaded = false;
        }
        //开启一个线程从磁盘读取数据
        new Thread("SharedPreferencesImpl-load") {
            public void run() {
                loadFromDisk();
            }
        }.start();
    }

    private void loadFromDisk() {
        synchronized (mLock) {
            //是否读取过
            if (mLoaded) {
                return;
            }
            //如果备份文件存在，删除mFile，将备份文件重命名给mFile
            if (mBackupFile.exists()) {
                mFile.delete();
                mBackupFile.renameTo(mFile);
            }
        }
        Map<String, Object> map = null;
        StructStat stat = null;
        Throwable thrown = null;
        try {
            stat = Os.stat(mFile.getPath());
            if (mFile.canRead()) {
                BufferedInputStream str = null;
                try {
                    str = new BufferedInputStream(
                            new FileInputStream(mFile), 16 * 1024);
                    //将xml文件转成map
                    map = (Map<String, Object>) XmlUtils.readMapXml(str);
                } catch (Exception e) {
                } finally {
                    IoUtils.closeQuietly(str);
                }
            }
        } catch (ErrnoException e) {
        } catch (Throwable t) {
            thrown = t;
        }

        synchronized (mLock) {
            //表示已经读取过，下次调用getSharedPreferences不会再从磁盘读取
            mLoaded = true;
            mThrowable = thrown;
            try {
                if (thrown == null) {
                    if (map != null) {
                        //赋值给成员变量mMap
                        mMap = map;
                        mStatTimestamp = stat.st_mtim;
                        mStatSize = stat.st_size;
                    } else {
                        mMap = new HashMap<>();
                    }
                }
            } catch (Throwable t) {
                mThrowable = t;
            } finally {
                //释放锁
                mLock.notifyAll();
            }
        }
    }

    @Override
    @Nullable
    public String getString(String key, @Nullable String defValue) {
        synchronized (mLock) {
            //阻塞等待sp将xml读取到内存后再get
            awaitLoadedLocked();
            String v = (String)mMap.get(key);
            //如果value为空返回默认值
            return v != null ? v : defValue;
        }
    }

    @GuardedBy("mLock")
    private void awaitLoadedLocked() {
        if (!mLoaded) {
            BlockGuard.getThreadPolicy().onReadFromDisk();
        }
        //sp读取完成后会把mLoaded设置为true
        while (!mLoaded) {
            try {
                mLock.wait();
            } catch (InterruptedException unused) {
            }
        }
        if (mThrowable != null) {
            throw new IllegalStateException(mThrowable);
        }
    }

（3）Editor源码

    @Override
    public Editor edit() {
        synchronized (mLock) {
            awaitLoadedLocked();
        }
        //每次都会返回一个新的Editor
        return new EditorImpl();
    }

    public final class EditorImpl implements Editor {
        //code...
        //将键值对写入mModified
        @Override
        public Editor putString(String key, @Nullable String value) {
            synchronized (mEditorLock) {
                mModified.put(key, value);
                return this;
            }
        }

        @Override
        public Editor putInt(String key, int value) {
            synchronized (mEditorLock) {
                mModified.put(key, value);
                return this;
            }
        }
        @Override
        public Editor putLong(String key, long value) {
            synchronized (mEditorLock) {
                mModified.put(key, value);
                return this;
            }
        }
        @Override
        public Editor putFloat(String key, float value) {
            synchronized (mEditorLock) {
                mModified.put(key, value);
                return this;
            }
        }
        @Override
        public Editor putBoolean(String key, boolean value) {
            synchronized (mEditorLock) {
                mModified.put(key, value);
                return this;
            }
        }

        @Override
        public Editor remove(String key) {
            synchronized (mEditorLock) {
                mModified.put(key, this);
                return this;
            }
        }

        @Override
        public Editor clear() {
            synchronized (mEditorLock) {
                //根据这个判断是否clear
                mClear = true;
                return this;
            }
        }

        @Override
        public void apply() {
            final long startTime = System.currentTimeMillis();

            final MemoryCommitResult mcr = commitToMemory();
            final Runnable awaitCommit = new Runnable() {
                    @Override
                    public void run() {
                        try {
                            mcr.writtenToDiskLatch.await();
                        } catch (InterruptedException ignored) {
                        }
                    }
                };

            QueuedWork.addFinisher(awaitCommit);

            Runnable postWriteRunnable = new Runnable() {
                    @Override
                    public void run() {
                        awaitCommit.run();
                        QueuedWork.removeFinisher(awaitCommit);
                    }
                };

            SharedPreferencesImpl.this.enqueueDiskWrite(mcr, postWriteRunnable);
            notifyListeners(mcr);
        }

        @Override
        public boolean commit() {
            long startTime = 0;

            if (DEBUG) {
                startTime = System.currentTimeMillis();
            }
            //提交到内存中，并返回mcr对象
            MemoryCommitResult mcr = commitToMemory();
            //放入写入队列
            SharedPreferencesImpl.this.enqueueDiskWrite(
                mcr, null /* sync write on this thread okay */);
            try {
                //线程等待，直到写入文件操作后
                mcr.writtenToDiskLatch.await();
            } catch (InterruptedException e) {
                return false;
            } finally {
            }
            //通知监听
            notifyListeners(mcr);
            //返回提交结果
            return mcr.writeToDiskResult;
        }

        private MemoryCommitResult commitToMemory() {
            long memoryStateGeneration;
            List<String> keysModified = null;
            Set<OnSharedPreferenceChangeListener> listeners = null;
            Map<String, Object> mapToWriteToDisk;

            synchronized (SharedPreferencesImpl.this.mLock) {
                if (mDiskWritesInFlight > 0) {
                    //这个时候说明正在写入文件，无法修改，写入文件之后会重置为0
                    mMap = new HashMap<String, Object>(mMap);
                }
                mapToWriteToDisk = mMap;
                mDiskWritesInFlight++;

                boolean hasListeners = mListeners.size() > 0;
                if (hasListeners) {
                    keysModified = new ArrayList<String>();
                    listeners = new HashSet<OnSharedPreferenceChangeListener>(mListeners.keySet());
                }

                synchronized (mEditorLock) {
                    boolean changesMade = false;
                    //调用了clear方法的操作
                    if (mClear) {
                        if (!mapToWriteToDisk.isEmpty()) {
                            changesMade = true;
                            mapToWriteToDisk.clear();
                        }
                        mClear = false;
                    }

                    for (Map.Entry<String, Object> e : mModified.entrySet()) {
                        String k = e.getKey();
                        Object v = e.getValue();
                        //this就是为了判断remove的
                        if (v == this || v == null) {
                            if (!mapToWriteToDisk.containsKey(k)) {
                                continue;
                            }
                            mapToWriteToDisk.remove(k);
                        } else {
                            if (mapToWriteToDisk.containsKey(k)) {
                                Object existingValue = mapToWriteToDisk.get(k);
                                //未发生修改跳过
                                if (existingValue != null && existingValue.equals(v)) {
                                    continue;
                                }
                            }
                            mapToWriteToDisk.put(k, v);
                        }

                        changesMade = true;
                        if (hasListeners) {
                            keysModified.add(k);
                        }
                    }
                    //将修改的map置空
                    mModified.clear();

                    if (changesMade) {
                        //当前提交到内存的次数
                        mCurrentMemoryStateGeneration++;
                    }

                    memoryStateGeneration = mCurrentMemoryStateGeneration;
                }
            }
            //将提交内存次数，发生修改的集合，监听，整个map封装成MemoryCommitResult
            return new MemoryCommitResult(memoryStateGeneration, keysModified, listeners,
                    mapToWriteToDisk);
        }
        //code...
    }

    private void enqueueDiskWrite(final MemoryCommitResult mcr,
                                  final Runnable postWriteRunnable) {
        final boolean isFromSyncCommit = (postWriteRunnable == null);

        final Runnable writeToDiskRunnable = new Runnable() {
                @Override
                public void run() {
                    synchronized (mWritingToDiskLock) {
                        //写入文件
                        writeToFile(mcr, isFromSyncCommit);
                    }
                    synchronized (mLock) {
                        //写入完成后减1
                        mDiskWritesInFlight--;
                    }
                    if (postWriteRunnable != null) {
                        postWriteRunnable.run();
                    }
                }
            };
        if (isFromSyncCommit) {
            boolean wasEmpty = false;
            synchronized (mLock) {
                wasEmpty = mDiskWritesInFlight == 1;
            }
            if (wasEmpty) {
                //执行写入文件
                writeToDiskRunnable.run();
                return;
            }
        }
        //apply操作才会执行
        QueuedWork.queue(writeToDiskRunnable, !isFromSyncCommit);
    }

修改值只会将其存到mModified的map中去，所以在编辑器中所做的所有更改都会批处理，直到我们调用commit或apply才会设置到mMap和xml文件中。

• SP文件不宜过大，如果SP文件需要存储的内容过多，可以根据不同的功能划分成多个文件；
• 如果可以的话尽可能早的调用getSharedPreferences,这样在调用put和get操作时，文件已经被读取到内存中了；
• 不要多次调用edit(), 应该调用一次edit(),因为每次调用edit()都会新建一个Editor;
• 不要多次调用commit()或apply()，如果多次存入值，应该在最后一次调用。